Implantable cardiac devices include devices which monitor heart activity and/or provide corrective therapy to the heart in the form of applied electrical energy. Examples of implantable cardiac devices include cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators. These devices are designed to be small in size and implanted beneath the skin of a patient. The device establishes electrical contact with the heart by one or more electrical leads having electrodes implanted within the heart, attached to the surface of the heart, or disposed subcutaneously and spaced from the heart.
Implantable cardiac devices commonly have the capability to communicate data with a device called an external programmer or other non-implanted device via a radio-frequency telemetry link. A clinician may use such an external programmer to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker are typically modified after implantation in this manner. Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data which may typically be telemetered from an implantable device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations.
Telemetry systems for implantable medical devices utilize radio-frequency energy to enable bidirectional communication between the implantable device and an external programmer. An exemplary telemetry system for an external programmer and a cardiac pacemaker is described in U.S. Pat. No. 4,562,841, issued to Brockway et al. and assigned to Cardiac Pacemakers, Inc., the disclosure of which is incorporated herein by reference. A radio-frequency carrier is modulated with digital information, typically by amplitude shift keying where the presence or absence of pulses in the signal constitute binary symbols or bits. The external programmer transmits and receives the radio signal with an antenna incorporated into a wand which can be positioned in proximity to the implanted device. The implantable device generates and receives the radio signal by means of an antenna that can be formed by a wire coil inside of the device housing.
Power for the device is supplied solely by a self-contained battery, and when the battery is exhausted a re-implantation procedure must be performed. Power for telemetry represents a significant overhead that shortens battery life. A typical bradycardia pacemaker, for example, may draw a total system current of 18 microamps with 15 microamps consumed by telemetry when the device is communicating with an external programmer. It is an objective of the present invention to provide a method and apparatus that enables more efficient telemetry.